Heart disease is the most significant health issue by several measures, and heart failure is one of our greatest medical challenges. Heart transplantation has remained the gold standard treatment option for severe heart failure. However, widespread applicability of heart transplantation is limited by a chronic, overwhelming shortage of donor organs. Cardiac tissue engineering has the potential to provide alternative solutions by creating additional treatment options. The largest barrier to the engineering of solid organs remains the deficiency of a vascular supply to support the three- dimensionality of the tissue constructs. To that end, we have developed a method to promote the development of an intrinsic blood supply to cardiac muscle tissue engineered in vivo. We isolated primary neonatal cardiac myocytes and suspended the cells in a fibrin gel within a vascularized 'chamber' in vivo, which promotes angiogenesis. We found that, by 4 weeks, the individual cells had remodeled to form functional 3- dimensional cardiac muscle which is supported by a well-formed vascular bed, and exhibited functional, histologic and phenotypic characteristics of mature myocardial tissue. This provided proof of concept for the engineering of functional, vascularized 3-D heart muscle in vivo. The work defined in this current proposal is focused on understanding the mechanics of the construct formation in vivo, as well as optimizing its functionality. We are interested in improving several key variables during tissue formation. We will optimize the seeding density, implantation time and size of the constructs. In addition, we will evaluate the feasibility of utilizing the model as a vascular conduit and evaluate its ability to act as a "pump" by producing intraluminal pressure with contraction, and ultimately flow, via the addition of valves. Completion of this study will allow for the formation of vascularized cardiac constructs with a significant improvement in performance metrics. In addition, we plan to demonstrate the in vivo development of a tissue engineered, single chambered cardiac pump, capable of effecting fluid flow through the construct. This will bring us closer to the ultimate goal of engineering clinically useful heart muscle. Heart disease is the number one health burden. The research proposed in this study focuses on engineering three dimensional heart muscle tissue within the body. This will provide a key step toward the ultimate goal of developing replacement tissues for people with heart failure. [unreadable] [unreadable] [unreadable]